Conventionally, high-efficiency III-V multi junction solar cells are formed by growing all of the component subcells lattice-matched to a single substrate. Unfortunately, this approach limits material choices and, hence, the band gaps available for solar cell design, thereby resulting in sub-optimal power conversion efficiencies.
In an alternative approach, each component subcell of a solar cell structure is grown on the most suitable substrate. Then, the subcells are integrated into a multi junction solar cell structure using wafer bonding techniques.
One known wafer bonding technique employs transparent metal oxides as a bonding agent. For example, indium tin oxide has been shown to have acceptable optical transparency, as well as good electrical conductivity. However, indium tin oxide has presented difficulties in achieving high quality bonds over a large surface area.
Other known wafer bonding techniques include using thin metallic interface layers or direct semiconductor-to-semiconductor bonding through heavily-doped, thick III-V interface layers.
Nonetheless, those skilled in the art continue to seek new wafer bonding techniques, including wafer bonding techniques that may be used in the construction of high-efficiency solar cells.